The method wherein a diffusion dye is released or formed imagewise by heat development and the resultant diffusion dye is transferred to a dye-fixing element is practically used. In this method, by changing the type of the dye-providing compound to be used or the type of the silver halide to be used, a negative dye image, as well as a positive dye image, can be obtained. More details are described in U.S. Pat. No. 4,500,626, U.S. Pat. No. 4 483 914, U.S. Pat. No. 4 503 137, U.S. Pat. No. 4 559 290, JP-A-58-149049 ("JP-A" means unexamined published Japanese patent application), JP-A-60-133449, JP-A-59-218443, JP-A-61-238056, EP-A-210 660 (A2), and the like.
With respect to the method for obtaining, particularly, a positive color image by heat development, various methods are proposed. For example, U.S. Pat. No. 4,559,290 describes a method wherein a so-called DRR compound (a diffusion-dye releasing-type redox compound) is used, which has been formed into an oxidized compound incapable of releasing a dye image. In this method, a reducing agent is oxidized in proportion to the exposure amount of a silver halide by heat development, and the above oxidized compound is reduced with the unoxidized remaining reducing agent, to cause a diffusion dye to be released, to form a positive color image. Further, U.S. Pat. No. 4,783,396 and the Journal of Technical Disclosure ("Kokai Giho") No. 87-6199 (Vol. 12, No. 22) describe heat-development color light-sensitive materials wherein use is made, as a compound capable of releasing a diffusion dye by the same mechanism, of a compound capable of releasing a diffusion dye by cleavage of the N-X bond, in which X represents for an oxygen atom, a nitrogen atom, or a sulfur atom, in a reducing fashion.
Moreover, as described, for example, in the October edition of "Eizo Joho" (issued on Oct. 1, 1993), edited by Ken Kuniyone and published by Sangyo Kaihatsu Kiko KK, recently the progress of computer graphics and the like is remarkable, and high-image-quality color printers (color hard copies) of various systems for outputting the image information thereof are developed. Among them, printers wherein heat-development color light-sensitive materials using silver halides are used, such as "FUJIX PICTOGRAPHY 3000" or "PICTROSTAT DIGITAL 400", trade names, manufactured by Fuji Photo Film Co., Ltd., are sold or made public.
Since these apparatuses carry out negative-positive conversion in a digital fashion, as the light-sensitive materials used therein, a conventional DRR compound can be used as it is, and the discrimination is excellent.
By using these light-sensitive materials, a high-quality dye image can be obtained in a short period of time. However, in recent years, it is desired to quicken the process further in the market. To shorten the processing time further, it is conceived to carry out the process at a higher temperature, but control of the period of the process under such a condition becomes difficult, leading to unevenness of the image or the like, sometimes.
On the other hand, the technique for accelerating development by addition of a reducing agent, such as phenidone, as an electron transport agent (an electron transferring agent), has long been known. Nevertheless, when the existing reducing agent is used, harmful effects occur, such as deterioration of the stability of light-sensitive materials, an increase in the density of the white background, and lowering in color separation.